When we’re on Earth, the magnetic field of our planet protects us from most of the harmful radiation that comes towards us from the Sun. However, up in space, you would be away from the protection of the Earth, and so astronauts can be exposed to much more radiation.
That’s a bad thing because the radiation can damage the cells in your body and affect how they reproduce – this can result in cancer. So, for astronauts, NASA calculates the maximum amount of radiation that they can receive to minimise the risk of cancer. This means there’s time limit on how long they can spend in space.
At the moment, this does present a problem for long missions, like those planned to Mars